1. Field of The Invention
The present invention relates generally to optical instruments for identifying groups of distant objects and, more particularly, to instruments for optically identifying constellations of stars by comparing their position and shape to representations of stars on a chart.
2. Related Art
The grouping of stars into named constellations is thousands of years old. For at least two thousand years, star charts have been used to assist in identifying constellations. One such early chart is the astrolabe. It has been used to determine one's location on the Earth and to learn to identify constellations and individual stars in them. Most modem star charts are simplifications of the ancient astrolabe, and have various names such as planisphere, star locator, and so on. These charts consist essentially of representations of stars from the visible sky as projected onto a flat disk. In some cases, a device is employed to adjust a display of the chart so that it shows the visible stars from a particular location at a particular time of year. This adjustment is done, for example, by viewing a generalized star chart through an elliptical window, the aperture of which shows the stars visible above the horizon at a particular time of night and day of the year. Such a chart typically is made to rotate around the celestial North or South Pole.
One common type of chart, related to the above mentioned adjustable ones, is called a zenith map. Zenith maps consist of a flat (usually azimuthal equidistant) projection of the starry hemisphere for a particular latitude on Earth and time of the year. The zenith is the center of the chart, and the horizon is portrayed as an outer circle around the zenith. Twelve such charts, one for each month, typically are deemed sufficient to serve through the year. Zenith maps have considerably less distortion than the projections used in the more generalized planisphere type star charts.
Notwithstanding the availability of zenith maps and other types of star charts, there are significant disadvantages to using them for learning the constellations, individual stars, and other astronomical subjects. According to a traditional technique, one looks at a star chart and then shifts focus to the sky in order to attempt to find the corresponding celestial subjects. Often, however, it is not clear if a subject found in the sky corresponds with a subject appearing on the chart. This difficulty occurs because it is necessary to continually shift attention between the chart and the stars in the sky and, typically, constellations as shown on the chart may not be the same size, or in the same orientation, as patterns of stars in the sky. Also, it typically is necessary to illuminate the star chart so that it may be seen at night. The illuminated star chart thus is usually significantly brighter than the sky. It therefore typically is necessary for the observer's eyes continually to adjust between the brighter light of the star chart and the dimmer light of the night sky. This adjustment introduces delay and makes it more difficult to relocate the group of stars that were being observed. Thus, this traditional procedure is discouraging to many.
Devices have been developed for the purpose of allowing a user to compare stars in the sky to a star chart without continually shifting attention between the sky and the chart. One type of such devices may be referred to as "binocular superposition" devices, an example of which is described in J. B. Caplan and H. J. Gerritsen, "Imaging the stars: two approaches to the development of a holographic star chart," SPIE Proceedings, Practical Holography X, 2652, pages 266-275 (1996) (hereafter referred to as "Caplan and Gerritsen"). The term "binocular" is employed to indicate that both eyes simultaneously are used to look through the device. In the device described in Caplan and Gerritsen, an observer uses both eyes to look at the sky through a beamsplitter. The beamsplitter combines images from a star chart and the night sky. By moving the star chart, the observer tries to obtain a superposition of the real sky and the map images.
There are serious disadvantages, however, to this approach. Using known holographic designs, the device described in Caplan and Gerritsen is relatively expensive and heavy. Obviously, the weight of such a device is important since it typically will be held by an observer for significant periods of time, and often at angles (such as when viewing near the zenith, for example) that make holding awkward or difficult. Significantly, the holographic device described in Caplan and Gerritsen also typically is limited to observing approximately a 20 degree portion of the sky. Such a restricted view limits the application of the device to perhaps one small constellation at a time.
Another type of binocular approach is described in Ben Mayer, Starwatch, Perigee Books, The Putnam Publishing Group, (1984), page 17, significant aspects of which appear to be incorporated in a product called "Star Finder" made by ALPI International, Oakland, Calif. The author describes painting stars as fluorescent dots on a transparent sheet. The observer attempts to superimpose these dots with real stars by holding the sheet at arms length and adjusting its direction and orientation. Using a sheet that is of a size and weight comfortable to hold at arms length, only a small portion of the sky may be represented on the sheet. Typically, this approach thus allows the study of only one constellation at a time. It therefore is not possible to move smoothly from one constellation to the next, because separate sheets must be used. Also, unless one views the constellations with only one eye, parallax problems result. That is, if one eye sees a star on the sheet superimposed with a star in the sky, the other eye sees that same star on the sheet superimposed with a totally different part of the sky, typically making an angle of about six degrees with the first direction. In addition it is difficult to adjust one's focus (so called accommodation) between the stars. Moreover, some people find that holding one's eye shut, or using a patch, for the length of time needed to locate a constellation is confining and uncomfortable.
U.S. Pat. No. 5,649,827 to Suzaki describes a large device, including a dome, in which the problems of parallax and accommodation appear to be ameliorated due to the larger distances involved. However, this device is not portable. The patterns are projected on the dome by a laser rather than being afixed to the dome.
A monocular device is also known that, like the device of Caplan and Gerritsen, employs a type of beamsplitter to join two images. However, it retains some of the disadvantages noted with respect to Caplan and Gerritsen: in particular, it is relatively heavy and may be relatively expensive. U.S. Pat. No. 5,311,203 to Norton, et al. describes a monocular instrument through which an observer may observe two images through one eye. One image is of subjects in the field of view of the instrument, such as stars in the sky. The other image is of an electronically generated reference display. These two images are joined by a beamsplitter so that a superimposed image is created. Light from the combined image then follows a common path to the observer's eye. (See, e.g., column 3, lines 40-55.) Thus, parallax problems are avoided, but the observer must hold the instrument containing the beamsplitter, electronics for generating the reference display, and other elements, such as positions sensors.
Yet another known type of monocular device presents a map of celestial subjects to one eye, but has serious disadvantages if one attempts to use it to correlate subjects seen on the map with subjects in the sky. This type of device is described in U.S. Pat. No. 5,003,698 to Vuarnesson, and aspects of the described device are included in a product called "Cosmic," also called "Stellarscope", that is produced by Sculptures-Jeux of Paris, France. The Cosmic device uses a small star map in which the stars are transparent dots on a black background. This map may be viewed with one eye with the use of a low magnification lens. The device portrays all of the visible stars in a single view. Thus, the angle between representations of any two given stars as viewed through the device is much smaller than the angle between the corresponding stars as viewed in the sky. Consequently, the representational stars appear to be much closer together than the actual stars in terms of angles. Thus, it may be difficult to correlate representational star patterns with actual star patterns.
Therefore, what is needed is a relatively light and inexpensive device for enabling an observer conveniently and reliably to identify distant subjects, such as stars, by comparing them with representations of those subjects, such as represented on a star map.